The present invention relates to a developing device for a copier, facsimile apparatus, printer or similar image forming apparatus. More particularly, the present invention is concerned with a developing device of the type transferring a developer deposited on a first developer carrier and leveled by a regulating member to a second developer carrier, and then feeding it to an image carrier in order to develop a latent image formed on the image carrier.
Generally, a developing system for an image forming apparatus uses either a one-ingredient type or a two-ingredient type developer. The one-ingredient type developer is magnetic toner having high electric resistance while the two-ingredient type developer is a mixture of nonmagnetic toner and carrier particles for depositing charge on the toner. Most of miniature laser printers extensively used today are implemented by the one-ingredient type developer or magnetic toner. A system using the magnetic toner is advantageous over a system using the toner and carrier mixture in that it reduces the cost and size of the apparatus. However, the problem with the magnetic toner scheme is that it is difficult to uniformly charge its particles, i.e., the toner includes particles charged to a polarity opposite to an expected polarity. The particles charged to the opposite polarity deposit on and smear, e.g., the non-image area or background of an image carrier.
In order to protect the background from contamination due to the above undesirable toner particles, Japanese Patent Laid-Open Publication No. 61-34557 discloses a developing system having a first and a second developer carrier. Magnetic toner charged by friction or charge injection is deposited on and conveyed by the first developer carrier and then transferred to the second developer carrier by an electric force. The second developer carrier conveys the toner to a developing position where it faces an image carrier. Japanese Patent Laid-Open Publication No. 6-222657 teaches an electrophotographic recording apparatus having a second developer carrier implemented as a belt and interposed between a first developer carrier and an image carrier. In these conventional schemes, among charged toner particles deposited on the first developer carrier, particles charged to a preselected polarity are transferred to the second developer carrier. As a result, the amount of particles of the opposite polarity and conveyed to the image carrier is successfully reduced. The background of the image carrier, therefore, suffers from a minimum of contamination attributable to the undesirable particles.
The above documents also teach that the second image carrier bite into the first developer carrier at least during the course of image formation. In this condition, the toner on the second developer carrier and varied in charge condition due to friction between it and the image carrier is stripped off by the first developer carrier at a nip between the two developer carriers. As a result, toner of the expected charge is fed to the second developer carrier and obviates defective images. The documents further teach that when the surface roughness of the first developer carrier is selected to be less than 10 .mu.m Rz inclusive in terms of a ten-point mean value as prescribed by JIS (Japanese Industrial Standards), the toner can be adequately charged and also eliminates defective images including an image with a smeared background.
However, experiments showed that when the same part of the surface of the second developer carrier runs past the nip between the first and second developer carriers a number of times without any developer being fed from the second developer carrier to the image carrier, the toner deposited on the second developer carrier sometimes cannot be sufficiently stripped off by the first developer carrier. Hence, the amount of toner on the second developer carrier sequentially increases. As a result, at the beginning of the subsequent image formation, an excessive amount of toner is fed to the image carrier and increases the density of the initially developed portion of an image.